1. Field of the Invention
The present invention generally relates to an optical writing device for use in an image forming apparatus such as a digital copier, a digital printer or a digital facsimile. More particularly, the present invention relates to an optical writing device in which multiple light beams, emitted by a plurality of light sources of a light source array, are focused onto a surface of a photosensitive medium without deflection. Further, the present invention relates to an image forming apparatus and method in which the optical writing device is used as an exposure unit that exposes the photosensitive medium surface to an imaging light pattern.
2. Description of the Related Art
With the widespread use of image forming systems, such as digital copiers, digital printers and digital facsimiles, there is an increasing demand for a small-size optical writing device for use in image forming systems.
There are two major types of optical writing device: a deflection type and a non-deflection type. In the deflection type, a rotary deflector or the like is provided to deflect the multiple light beams, emitted by a plurality of light sources of a light source array (for example, a semiconductor laser array), and the deflected light beams are focused onto the surface of the photosensitive medium. In the non-deflection type, the light beams, emitted by the light source array, are focused onto the surface of the photosensitive medium without deflection.
The disadvantage of the deflection type is that a total length of the optical path in the optical writing device becomes large because of the use of the rotary deflector, which is not suitable to provide a small-size optical writing device. On the other hand, the non-deflection type does not use a rotary deflector and can shorten the total length of the optical path, and, therefore, it is more suitable to provide a small-size optical writing device. Moreover, the non-deflection type optical writing device does not require mechanical drive parts that move a rotary deflector, and it can provide a low-cost optical writing device.
A conventional optical writing device is known, which is of the non-deflection type and uses a rod lens array as the means for focusing the light beams, emitted by the light source array, onto the photosensitive medium surface. FIG. 31 shows a distribution of light amount of a rod lens array in the conventional optical writing device. FIG. 32 shows a relationship between the rod lens diameter and the visual field radius in the conventional rod lens array of FIG. 31.
As shown in FIG. 31 and FIG. 32, the rod lens array 110 in the conventional optical writing device includes a plurality of rod lens elements 101a that focuses the light beams from a light source array onto an image plane. These rod lens elements 101a are arrayed in a row in an array direction. Each of the rod lens elements 101a has a distribution of light amount due to a distributed refractive index of each rod lens element. Respective images, which are formed on the image plane by the light beams passed through the rod lens elements 101a are overlapped each other in the array direction so as to form a line-shaped image. As the light amount distributions of the respective lens elements are superimposed, the distribution of light amount of the conventional rod lens array 101 in the array direction of the rod lens elements 101a is as shown in FIG. 31. For this reason, the light amount distribution of the conventional rod lens array is liable to the periodic variations of light amount which depend on the visual field radius of each rod lens element and the pitch of the rod lens elements in the array direction. The magnitude xcex94E of the periodic variations of the light amount, caused by the conventional rod lens array 101, is represented by the following formula:
xcex94E=(Emaxxe2x88x92Emin)/Emaxxc3x97100 (%)xe2x80x83xe2x80x83(1)
where Emin is the minimum light amount in the superimposed distribution and Emax is the maximum light amount in the superimposed distribution. In FIG. 32, xe2x80x9cDxe2x80x9d indicates the rod lens diameter and xe2x80x9cXxe2x80x9d indicates the visual field radius in the conventional rod lens array 101 of FIG. 31.
Generally, the periodic variations of light amount in the conventional optical writing device depend on the visual field radius of each rod lens element and the pitch of the rod lens elements in the array direction. Herein, it is assumed that the conventional rod lens array is constituted by identical rod lens elements which are arrayed in a row in the array direction, and that all of respective pitches of two adjacent ones of the individual rod lens elements in the array direction are nearly equal to the diameter of each rod lens element in the array direction. Further, it is assumed that the conventional rod lens array includes only the rod lens elements and does not include shading portions between the rod lens elements. Typically, in the conventional optical writing devices, the visual field radius X of each rod lens element in the array direction is on the order of 1 to 2 mm, and the diameter D of each rod lens element in the array direction is approximately 1 mm. Specifically, in the example of the conventional rod lens array 101 of FIG. 32, D=1 mm, X=1.5 mm.
Japanese Laid-Open Patent Application No.10-309826 discloses an optical writing device that uses a semiconductor laser array as a light source array for emitting multiple light beams. The semiconductor laser array used by this conventional device is, for example, an array of light emitting diodes (LED).
In the conventional device of the above document, a rod lens array is provided for focusing the light beams, emitted by the LED array, onto the photosensitive medium surface. In the rod lens array, the rod lens elements are arrayed in two rows in the array direction, and the lens elements of one row are spaced apart from the lens elements of the other row by a given pitch.
Hereinafter, throughout the specification, in order to represent a configuration of a focusing lens array, such as a rod lens array, an overlap ratio m is used, which is defined by the equation m=X/D where X indicates the visual field radius of each focusing lens element in the array direction and D indicates the diameter of each focusing lens element in the array direction.
In the conventional optical writing device of the above document, an overlap ratio m of each rod lens element of the rod lens array is defined by the equation m=X0/D where X0 indicates the visual field radius of each of the rod lens elements in the array direction and D indicates the diameter of each of the rod lens elements in the array direction. The conventional device of the above document is characterized in that the rod lens array is configured such that the overlap ratio m of the rod lens array satisfies the conditions 1.85 less than m less than 2.00. This configuration is selected by the conventional device in order to eliminate the undesired variations of the sub-scanning direction alignment of the light source array and the rod lens array.
However, the conventional device of the above document is liable to having the periodic variations of light amount of the rod lens array due to the configuration of the rod lens elements having a small overlap ratio. It is known from practical experience that the magnitude xcex94E of the periodic variations of the light amount is in a range from 10% to 20%. An image forming apparatus using such optical writing device will produce the periodic variations of photographic density in a reproduced image due to the periodic light amount variations of the rod lens array, and it is difficult for the image forming apparatus to provide the reproduced image with good quality.
It is an object of the present invention to provide an improved optical writing device in which the above-described problems are eliminated.
Another object of the present invention to provide an improved optical writing device that effectively reduces the periodic variations of light amount caused by the lens elements of the focusing lens array as in the conventional optical writing device.
Another object of the present invention is to provide an image forming apparatus which uses an optical writing device, the optical writing device being configured to effectively reduce the periodic variations of light amount caused by the lens elements of the focusing lens array as in the conventional optical writing device.
Another object of the present invention is to provide an image forming method which uses an optical writing device, the optical writing device being configured to effectively reduce the periodic variations of light amount caused by the lens elements of the focusing lens array as in the conventional optical writing device.
The above-mentioned objects of the present invention are achieved by an optical writing device comprising: a light source array which has an array of light sources emitting a plurality of light beams; and a focusing lens array which has a row of focusing lens elements focusing the light beams from the light source array onto a surface of a photosensitive medium, the focusing lens elements being arrayed in an array direction, each focusing lens element having a visual field radius X in the array direction and a diameter D in the array direction, wherein the focusing lens array is configured to satisfy the condition: m greater than 2.0 where m is an overlap ratio of each of the focusing lens elements defined by the equation m=X/D.
The above-mentioned objects of the present invention are achieved by an optical writing device comprising: a light source array which has an array of light sources emitting a plurality of light beams; a focusing lens array which has a row of focusing lens elements focusing the light beams from the light source array onto a surface of a photosensitive medium, the focusing lens elements being arrayed in an array direction, each focusing lens element having a visual field radius Xxe2x80x2 in the array direction and an aperture diameter d in the array direction; and a plurality of shading portions which are disposed between the focusing lens elements of the focusing lens array, wherein the focusing lens array is configured to satisfy the condition: mxe2x80x2 greater than 2.0 where mxe2x80x2 is an overlap ratio of each of the focusing lens elements defined by the equation mxe2x80x2=Xxe2x80x2/d.
The above-mentioned objects of the present invention are achieved by an optical writing device comprising: a light source array which has an array of light sources emitting a plurality of light beams; and a focusing lens array which has a plurality of rows of focusing lens elements focusing the light beams from the light source array onto a surface of a photosensitive medium, the focusing lens elements being arrayed in an array direction, each focusing lens element having a visual field radius X in the array direction and a diameter D in the array direction, wherein the focusing lens array is configured to satisfy the condition: m greater than 2.0 where m is an overlap ratio of each of the focusing lens elements defined by the equation m=X/D.
The above-mentioned objects of the present invention are achieved by an image forming apparatus comprising: an optical writing device; and a photosensitive medium which has a surface on which an electrostatic latent image is formed by exposing the surface to light emitted and focused by the optical writing device, wherein the optical writing device comprises: a light source array which has an array of light sources emitting a plurality of light beams; and a focusing lens array which has a row of focusing lens elements focusing the light beams from the light source array onto the surface of the photosensitive medium, the focusing lens elements being arrayed in an array direction, each focusing lens element having a visual field radius X in the array direction and a diameter D in the array direction, wherein the focusing lens array is configured to satisfy the condition: m greater than 2.0 where m is an overlap ratio of each of the focusing lens elements defined by the equation m=X/D.
The above-mentioned objects of the present invention are achieved by an image forming method which comprises the steps of: providing an optical writing device, the optical writing device comprising a light source array having an array of light sources emitting a plurality of light beams, and a focusing lens array having a row of focusing lens elements focusing the light beams from the light source array onto a surface of a photosensitive medium; controlling the light source array to emit the light beams; and forming an electrostatic latent image on the surface of the photosensitive medium by exposing the photosensitive medium surface to the light beams focused by the focusing lens array, wherein the focusing lens elements of the optical writing device are arrayed in an array direction, each focusing lens element having a visual field radius X in the array direction and a diameter D in the array direction, wherein the optical writing device is configured to satisfy the condition: m greater than 2.0 where m is an overlap ratio of each of the focusing lens elements defined by the equation m=X/D.
In the optical writing device of the present invention, the focusing lens array is configured to satisfy the condition: m greater than 2.0 where m is an overlap ratio of each of the focusing lens elements defined by the equation m=X/D, X indicates the visual field radius of each focusing lens element in the array direction, and D indicates the diameter of each focusing lens element in the array direction. Therefore, the optical writing device of the present invention is effective in reducing the periodic variations of light amount caused by the focusing lens elements of the conventional optical writing device.
In the image forming apparatus and method of the present invention, the optical writing device in which the focusing lens array is configured to satisfy the condition: m greater than 2.0 is provided. As the periodic variations of light amount, caused by the focusing lens elements of the conventional optical writing device, are eliminated, the image forming apparatus and method of the present invention can create a reproduced image with good quality and eliminate the periodic variations of photographic density in the reproduced image.